A vehicle may contain automatic safety restraint actuators which are activated responsive to a vehicle crash for purposes of mitigating occupant injury. Examples of such restraint actuators include air bags, seat belt pretensioners, and deployable knee bolsters.
One objective of an automatic safety restraint system is to mitigate occupant injury, thereby not causing more injury with the automatic restraint system than would be caused by the crash had the automatic restraint system not been activated. Notwithstanding the protective benefit of these automatic safety restraint actuators, there is generally both a risk and a cost associated with the deployment thereof. Generally, it is desirable to only activate automatic safety restraint actuators when needed to mitigate injury because of the expense of replacing the associated components of the safety restraint system, and because of the potential for such activations to harm occupants. This is particularly true of air bag restraint systems, wherein occupants too close to the air bag at the time of deployment--i.e. out-of-position occupants--are vulnerable to injury or death from the deploying air bag even when the associated vehicle crash is relatively mild. Moreover, occupants who are of small stature or with weak constitution, such as children, small adults or people with frail bones are particularly vulnerable to injury induced by the air bag inflator. Furthermore, infants properly secured in a normally positioned rear facing infant seat (RFIS) in proximity to a front seat passenger-side air bag are also vulnerable to injury or death from the deploying air bag because of the close proximity of the infant seat's rear surface to the air bag inflator module.
While air bags are designed to protect vehicle occupants, conventional crash detection and safety restraint deployment systems only use sensors which are mounted on the vehicle frame and are triggered by acceleration or velocity of the car rather than the occupant. Accordingly, conventional deployment strategies are not directly based on the weight, stature, and position of vehicle occupants. It is often very difficult to discriminate between crashes where air bags should be deployed and when their deployment could cause more harm than benefit. This difficult decision is typically made using only one or as few as possible sensors mounted on the vehicle. In the future, more occupant safety strategies will be available, including seat belt pre-tensioning and multi-stage air bags. With more available options, the deployment decision will become more complicated and require additional real-time occupant position data.
Air bag inflators are designed with a given restraint capacity, as for example, the capacity to protect an unbelted normally seated fiftieth percentile occupant when subjected to a 30 MPH barrier equivalent crash, which results in associated energy and power levels which can be injurious to out-of-position occupants. While relatively infrequent, cases of injury or death caused by air bag inflators in crashes for which the occupants would have otherwise survived relatively unharmed have provided the impetus to reduce or eliminate the potential for air bag inflators to injure the occupants which they are intended to protect.
One technique for mitigating injury to occupants by the air bag inflator is to reduce the power and energy levels of the associated air bag inflator, for example by reducing the amount of gas generant in the air bag inflator, or the inflation rate thereof. This reduces the risk of harm to occupants by the air bag inflator while simultaneously reducing the restraint capacity of the air bag inflator, which places occupants a greater risk for injury when exposed to higher severity crashes.
Another technique for mitigating injury to occupants by the air bag inflator is to control the rate of inflation rate or the capacity of the inflator responsive to a measure of the severity of the crash. However, the risk of injury to such occupants would not be mitigated under the conditions of higher crash severity when the inflator is intentionally made aggressive in order to provide sufficient restraint for normally positioned occupants.
Yet another technique for mitigating injury to occupants by the air bag inflator is to control the activation of the air bag inflator responsive to the presence, position, and size of the occupant, or to the severity of the crash. For example, the air bag inflator can be disabled if the occupant weight is below a given threshold. Moreover, the inflation capacity can be adjusted by controlling the number of inflation stages of a multi-stage inflator that are activated. Furthermore, the inflation power can be adjusted by controlling the time delay between the firings of respective stages of a multi-stage inflator.
One measure of restraint capacity of an air bag inflator is the amount of occupant kinetic energy that can be absorbed by the associated air bag system, whereby when the occupant collides with the gas filled air bag, the kinetic energy of the occupant is converted to potential energy via the pressurization of the air bag, and this potential energy is dissipated by venting pressurized gases from the air bag. As a vehicle in a crash is decelerated, the velocity of an unrestrained occupant relative to the vehicle increases. Preferably, the occupant restraint process is commenced early in the crash event so as to limit the amount of occupant kinetic energy which must be absorbed and thereby minimize the associated restraint forces and accelerations of and loads within the occupant. If the occupant were a simple inertial mass without friction relative to the vehicle, the kinetic energy of the occupant would be given by 1/2 M.multidot.V.sup.2, where M is the mass of the occupant and V is the occupant velocity relative to the vehicle. If a real occupant were represented by an interconnected set of bodies, some of which have friction relative to the vehicle, each body of which may have differing velocities relative the vehicle, the above equation would apply to the motion of the center of gravity of the occupant. Regardless of the representation, occupants of larger mass will have a larger kinetic energy for the same velocity relative to the vehicle. Therefore, an occupant weight sensor is useful in an air bag system with variable restraint capacity to enable the restraint capacity to be preferentially adapted to the weight, or mass, of the occupant.
Except for some cases of oblique or side-impact crashes, it is generally desirable to not activate an automatic safety restraint actuator if an associated occupant is not present because of the otherwise unnecessary costs and inconveniences associated with the replacement of a deployed air bag inflation system. Occupant presence can be detected by a seat weight sensor adapted to provide either a continuous measure of occupant weight or to provide a binary indication if the occupant weight is either above or below a specified weight threshold.
Known seat weight sensors comprise one or more pads employing force sensitive resistive (FSR) films. These arrangements are typically used as weight threshold systems to disable a passenger air bag when the seat is empty. Load cells attached to the seat mounting posts have also been used in research applications. Mechanisms which use string based potentiometers to measure downward seat displacement have also been investigated.
Such known arrangements suffer from several drawbacks. First, variable resistance force sensors have limited sensitivity and in some situations are not sensitive enough to put directly under a seat pad while still achieving the desired response. Second, the threshold weight system provides only very limited information. For example, such arrangements provide no indication as to the size of an occupant. Third, the resistance values of known variable force resistor change with temperature, and are subject to drift over time with a constant load on the sensor.
Furthermore, other known sensing arrangements do not otherwise provide suitable results. For example, the use of load cells is prohibitively expensive for large-scale commercial applications. Strain gauges of any type may be impractical because of the difficulty in applying them to the strained material. Mechanical string potentiometer based weight sensors are complex, and subject to failure from stretching of the string. String potentiometer based weight sensors also suffer from a limitation whereby seat geometry changes over the lifetime of the seat. More specifically, seats tend to take a "set" over time so that the springs and cushion tend to move downward as the seat ages. A string potentiometer based weight sensor measuring downward displacement would require periodic recalibration over the lifetime of the seat. Finally, optical or infrared sensors have been used to measure the spatial position of occupants relative to the dashboard or headliner. Often these sensors are also integrated with speed sensors to discern changes in occupant position due to car acceleration. Current optical and infrared occupant position sensors require augmented information from speed and weight sensors, thereby resulting in a relatively high cost distributed system which may be difficult to manufacture, install, and maintain. Furthermore, optical and/or infrared sensors which measure the range from the headliner or dashboard can be confused by placement of objects in front of an occupant, such as when reading newspapers or books, or by the position of the seat back because many seats can recline fully back and incline fully forward. Moreover, the sensing aperture of these sensors may become occluded by inadvertent scratching or substance application.
Known seat weight sensing techniques generally require multiple points for sensing distributed weight accurately. Also, force sensing resistors, load cells or membrane switches may require significant seat redesign for use in current or future seats. This is particularly true for spring type seats which do not provide a uniform horizontal support surface. The response time of load cells or membrane switches may fast enough for real-time applications.
The prior art also teaches the use of seat weight sensors outside the automotive environment, for example as a means for disabling the activation of either a boat or an industrial machine if the operator is not properly seated, or for weighing a person seated on an exercise bike. These devices employ pneumatic bladders located in the seat, whereby the pressure within the bladder is used to either activate a threshold switch or to provide a continuous indication of occupant weight.
One problem with prior art pneumatic sensors, particularly when applied to the automotive environment, is their sensitivity to environmental conditions, particularly to ambient temperature and pressure. This requires the bladder to be partially filled with fluid under ambient conditions of lower temperature or higher pressure, thereby making the bladder more susceptible to bottoming out when exposed to localized or concentrated loads and therefor requiring a means for distributing the loads over the load bearing area of the bladder. Pneumatic seat weight sensors can be sensitive to the amount of air initially in the associated bladder. A seat weight sensor in an automotive environment must function reliably and accurately over a wide range of temperatures and pressures which can cause significant errors.
The prior art also teaches the use of hydraulic load cells, wherein the weight to be measured acts upon a piston element of known area, whereby the measured weight is found by multiplying a measured pressure times the known area. One problem with hydraulic load cells in the automotive environment, particularly in a seat, is that the effects of load cell orientation on hydraulic head can introduce load measurement errors.
Application ASL-157-US discloses a fluid-filled seat weight sensor, one embodiment of which comprises a pneumatic seat weight sensor comprising a gas filled bladder mounted in the seat, a means for distributing the weight to be measured over the surface of the bladder, and a means for indicating the weight on the seat by measuring the pressure within the bladder relative to the ambient pressure. The pneumatic seat weight sensor may further comprise a means for refilling the gas within the gas-filled bladder to account for losses over time.
The gas-filled bladder is preferably only partially filled to allow for gaseous expansion due to variations in ambient temperature and pressure, such that over the possible range of environmental operating conditions the volume of the unloaded gas-filled bladder generally does not exceed the design volume thereof. Moreover, under these conditions, the associated absolute pressure in the bladder would not exceed ambient pressure.
Under the action of a distributed load, the volume of the bladder decreases until the pressure therein is sufficiently great to support the load. For a bladder having a design shape of a rectangular slab having a height and two base dimensions, as the height decreases under the action of the load, the base dimensions increase, thereby increasing the base area of the bladder. The weight of the distributed load is then given by the product of the base area of the bladder times the difference in pressure inside and outside the bladder. Even if the loading on the top of the seat is relatively localized, the associated weight is given by the differential pressure acting on the base area of the bladder, assuming the base of the bladder is fully supported and that that top surface of the bladder is not locally compressed against the bottom surface.
As noted above, the bladder is preferably only partially filled under nominal ambient conditions. Therefore, the action of a concentrated load on the bladder would most likely cause the top surface of the bladder to bottom out on the bottom surface. This prevents a portion of the load from being supported by the gas within the bladder so that the corresponding differential pressure measurement would not properly indicate the full weight on the bladder. This condition can be alleviated by providing a means for distributing the load across the bladder, such as with the foam pad constituting the seat cushion, or the system and method disclosed in Application ASL-186-US.
Generally, the sensitivity of the gas filled bladder to ambient temperature and pressure is decreased with decreasing amounts of gas in the bladder, and with decreasing bladder thickness for the same base dimensions of the bladder. However, as the bladder is made thinner in overall height, and the amount of gas is reduced, the bladder becomes more susceptible to bottoming-out under the influence of localized loads applied to the seat.
The gas-filled bladder may be of sealed construction with a fixed initial amount of gas. Alternately, the bladder may be equipped with a filling valve to refill gas that is lost to either osmosis or leakage, for example in accordance with Application ASL-163-US. Furthermore, the bladder may be equipped with a means to automatically refill this lost gas with the preferable amount of gas relative to the design volume of the bladder, generally about 30% to 50% of the design volume, and more particularly about 40%; for example in accordance with Applications ASL-186-US or ASL-187-US.
When incorporating a means for automatically refilling the bladder, the amount of gas in the bladder at any given time would likely not be known. The weight on the sensor is given by the expression W=DP/A, where DP is the differential pressure between the inside and outside of the bladder, and A is the base area of the bladder. The effect of the base area A of a partially filled bladder increasing with increasing load is included in the calibration. This effect is smaller for relatively thinner bladders, and is relatively insensitive to the fill conditions of the bladder.
If a pneumatic hydrostatic weight sensor not maintained in the proper partially filled state, then under some conditions, such as when the pneumatic hydrostatic weight sensor is used at a sufficiently greater temperature or reduced pressure relative to the corresponding temperature or pressure during fill conditions, then the gas in the gas-filled bladder can expand to exceed the design volume thereof, whereby a portion of the force resulting from the gas pressure therein is counteracted by internal membrane forces of the bladder. Consequently, under these conditions a resulting measure of the pressure differential across the gas-filled bladder would not entirely correlate with the applied weight so that a measure of weight based upon this differential pressure would not accurately indicate the amount of weight applied to the seat.
Another source of pneumatic hydrostatic weight sensor measurement error are alternate load paths by which a portion of the weight applied to the seat is supported by some means other than the pressure in the gas-filled bladder. For example, if the load bearing area of the gas-filled bladder were smaller than that of the seat, then those portions of the seat assembly adjacent the gas-filled bladder provide an alternate load path. As another example, media or mechanisms incorporated in the gas-filled bladder to restore and regulate the volume of gas therein also provides an alternate load path when the weight applied to the gas-filled bladder is sufficiently great to compress the volume restoring media or mechanisms.
One possible failure mode of a pneumatic hydrostatic weight sensor is the loss of gas from the gas-filled bladder, whereby corresponding differential pressure measurement would always indicate zero applied weight. Accordingly, the gas containing elements of the pneumatic hydrostatic weight sensor must be constructed ruggedly so as to work reliably over the expected life span. Moreover, the reliability of the associated system incorporating the pneumatic hydrostatic weight sensor could be enhanced by incorporating a separate means to detect whether the gas-filled bladder is evacuated.